A fluid warming cassette for use in a fluid warming system includes a first sheet, a second sheet and a flexible spacer having a serpentine opening. At least one of the first and second sheets is a flexible plastic sheet. The spacer is positioned between the first and second sheets and the first and second sheets are joined together over the spacer to form a fluid container having a fluid channel. The fluid container includes a fluid channel with inlet and outlet ports in fluid communication with the fluid channel. The fluid warming cassette is capable of operating under negative pressure without collapse of the fluid channel in the cassette.